Drop tracking misses variables consumed in let statements

[tmiasko]

For example when compiling with nightly-2022-01-22 (which included drop tracking):

#![feature(generators)]
#![feature(negative_impls)]

struct NotSend;
impl !Send for NotSend {}

fn assert_send<T: Send>(_: T) {}

fn main() {
    // Fails:
    assert_send(|| {
        let a = NotSend;
        let b = a; // a should be consumed here
        drop(b);
        yield;
    });
    // Succeeds:
    assert_send(|| {
        let a;
        a = NotSend;
        let b;
        b = a;
        drop(b);
        yield;
    });
}

This seems to be a mismatch between record_consumed_borrow::ExprUseDelegate which records a consume of a but associates it with a let statement let b = a;, while cfg_build::DropRangeVisitor considers only variables consumed by expressions.

[eholk]

Thanks for the test case! I moved this to the On Deck board. I should be able to get to it this week, but I'll leave it unclaimed in case someone else wants to get there first.

@rustbot label +AsyncAwait-Triaged +AsyncAwait-Polish

[rustbot]

Error: Label AsyncAwait-Triaged can only be set by Rust team members

Please let @rust-lang/release know if you're having trouble with this bot.

[eholk]

@rustbot label +AsyncAwait-Triaged +AsyncAwait-Polish

[vincenzopalazzo]

Ok I think I am missing some context here, and do not have the error that was discussed
it is a little bit confusing.

So with the following example

#![feature(generators)]
#![feature(negative_impls)]

struct NotSend;
impl !Send for NotSend {}

fn assert_send<T: Send>(_: T) {}

fn main() {
    // Fails:
    assert_send(|| {
        let a = NotSend;
        let b = a; // a should be consumed here
        drop(b);
        yield;
    });
    // Succeeds:
    assert_send(|| {
        let a;
        a = NotSend;
        let b;
        b = a;
        drop(b);
        yield;
    });
}

The error generated is the following on with the commit 136d74f

➜  rust git:(macros/112850) ✗ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc test.rs --edition 2021
error: generator cannot be sent between threads safely
  --> test.rs:11:17
   |
11 |       assert_send(|| {
   |  _________________^
12 | |         let a = NotSend;
13 | |         let b = a; // a should be consumed here
14 | |         drop(b);
15 | |         yield;
16 | |     });
   | |_____^ generator is not `Send`
   |
   = help: within `{generator@test.rs:11:17: 11:19}`, the trait `Send` is not implemented for `NotSend`
note: generator is not `Send` as this value is used across a yield
  --> test.rs:15:9
   |
12 |         let a = NotSend;
   |             - has type `NotSend` which is not `Send`
...
15 |         yield;
   |         ^^^^^ yield occurs here, with `a` maybe used later
16 |     });
   |     - `a` is later dropped here
note: required by a bound in `assert_send`
  --> test.rs:7:19
   |
7  | fn assert_send<T: Send>(_: T) {}
   |                   ^^^^ required by this bound in `assert_send`

error: generator cannot be sent between threads safely
  --> test.rs:18:17
   |
18 |       assert_send(|| {
   |  _________________^
19 | |         let a;
20 | |         a = NotSend;
21 | |         let b;
...  |
24 | |         yield;
25 | |     });
   | |_____^ generator is not `Send`
   |
   = help: within `{generator@test.rs:18:17: 18:19}`, the trait `Send` is not implemented for `NotSend`
note: generator is not `Send` as this value is used across a yield
  --> test.rs:24:9
   |
19 |         let a;
   |             - has type `NotSend` which is not `Send`
...
24 |         yield;
   |         ^^^^^ yield occurs here, with `a` maybe used later
25 |     });
   |     - `a` is later dropped here
note: required by a bound in `assert_send`
  --> test.rs:7:19
   |
7  | fn assert_send<T: Send>(_: T) {}
   |                   ^^^^ required by this bound in `assert_send`

error: aborting due to 2 previous errors

@rustbot claim

[vincenzopalazzo]

Mh looking at the error looks like the following help message is wrong?

   = help: within `{generator@test.rs:11:17: 11:19}`, the trait `Send` is not implemented for `NotSend`
note: generator is not `Send` as this value is used across a yield
  --> test.rs:15:9
   |
12 |         let a = NotSend;
   |             - has type `NotSend` which is not `Send`
...
15 |         yield;
   |         ^^^^^ yield occurs here, with `a` maybe used later
16 |     });
   |     - `a` is later dropped here
note: required by a bound in `assert_send`
  --> test.rs:7:19
   |
7  | fn assert_send<T: Send>(_: T) {}
   |                   ^^^^ required by this bound in `assert_send`

[tmiasko]

Note that the drop tracking is gated by -Zdrop-tracking.

The test case was fixed by -Zdrop-tracking-mir. Closing as fixed.